The bcl-3 Proto-Oncogene Encodes a Nuclear IκB-like Molecule That Preferentially Interacts with NF-κB p50 and p52 in a Phosphorylation-Dependent Manner

Abstract

The product of the putative proto-oncogene bcl-3 is an IκB-like molecule with novel binding properties specific for a subset of the rel family of transcriptional regulators. In vitro, Bcl-3 protein specifically inhibited the DNA binding of both the homodimeric NF-κB p50 subunit and a closely related homolog, p52 (previously p49), to immunoglobulin κNF-κB DNA motifs. Bcl-3 could catalyze the removal of these proteins from DNA. At concentrations that significantly inhibited DNA binding by homodimeric p50, Bcl-3 did not inhibit binding of reconstituted heterodimeric NF-κB (p50:p65), a DNA-binding homodimeric form of p65, or homodimers of c-Rel. Phosphatase treatment of Bcl-3 partially inactivated its inhibitory properties, implicating a role for phosphorylation in the regulation of Bcl-3 activity. Bcl-3, like p50, localizes to the cell nucleus. In cells cotransduced with Bcl-3 and p50, both molecules could be found in the nucleus of the same cells. Interestingly, coexpression of Bcl-3 with a p50 mutant deleted for its nuclear-localizing signal resulted in the relocalization of Bcl-3 to the cytoplasm, showing that the proteins interact in the cell. These properties contrast Bcl-3 to classically defined IκB, which maintains heterodimeric NF-κB p50:p65 in the cytoplasm through specific interactions with the p65 subunit. Bcl-3 appears to be a nuclear, IκB-related molecule that regulates the activity of homodimeric nuclear p50 and its homolog p52.